This invention relates to a non-aqueous electrolyte to be used for secondary batteries and also to a non-aqueous electrolyte secondary battery using the same.
Non-aqueous electrolyte lithium ions and lithium secondary batteries formed by using a carbon material, an oxide, a lithium alloy or lithium metal for the negative electrode have been attracting attention as power sources for portable telephone sets, notebook type personal computers and so on because they can realize a high energy density. It is known that a film referred to as surface film, protective film or solid electrolyte interface (SEI) or simply as film (to be referred to as “surface film” hereinafter) tends to be formed on the surface of the negative electrode. It is also known that such a surface film indispensably needs to be rigorously controlled to improve the performance of the negative electrode because it significantly influences the charge-discharge efficiency, the cycle life and the safety of the battery. As for prior art secondary batteries of the type under consideration, the irreversible capacity of the carbon material or the oxide material of the negative electrode needs to be reduced and, in the case of a lithium metal negative electrode or a lithium alloy negative electrode, both the problem of fall of the charge-discharge efficiency and that of safety due to generation of dendrite need to be solved.
Various techniques have been proposed to dissolve the above-identified problems. For example, there are proposed techniques for suppressing generation of dendrite by arranging a film layer of lithium fluoride on the surface of the lithium metal or the lithium alloy of the negative electrode by means of a chemical reaction.
For instance, JP-A-7-302617 discloses a technique of covering the surface of a lithium negative electrode with a lithium fluoride film by exposing it to an electrolyte containing hydrofluoric acid and causing the negative electrode to react with hydrofluoric acid. Hydrofluoric acid is produced as a result of reaction of LiPF6 and a minute quantity of water. On the other hand, a surface film of lithium hydroxide or lithium oxide is formed on the surface of the lithium negative electrode due to natural oxidation in the air. A surface film of lithium fluoride is formed on the surface of the negative electrode as they react with each other. However, such a lithium fluoride film is produced as a result of reaction of the electrode interface and a solution and hence one or more than one side reaction products are apt to be mingled in the surface film to make it difficult to obtain a homogeneous film. Additionally, there may be occasions where the surface film of lithium hydroxide or lithium oxide is not formed homogeneously and those where lithium is partly exposed. In such occasions, no homogeneous thin film can be formed and water or hydrogen fluoride and lithium may react with each other to give rise to problems. If the reaction is insufficient, unnecessary compounds are left in addition to the fluorides to consequently reduce the ion conductivity. Furthermore, the scope of fluorides and that of electrolytes that can be utilized are limited and it is difficult to form a stable surface film at a high yield rate with such a technique of forming a fluoride layer by means of a chemical reaction on the interface.
JP-A-8-250108 describes a technique of forming a surface film of lithium fluoride on the surface of a negative electrode by causing a mixture gas of argon and hydrogen fluoride and an aluminum-lithium alloy to react with each other. However, the reaction can be easily become uneven to make it difficult to form a homogeneous lithium fluoride film when a surface film exists in advance on the surface of the lithium metal and particularly when a plurality of different compounds exists there. Therefore, it is difficult to obtain a lithium secondary battery having sufficient cycle characteristics.
JP-A-11-288706 discloses a technique of forming a surface film structure containing a substance having a halite-type crystal structure as principal ingredient on the surface of a lithium sheet of a uniform crystal structure where preferential orientation of the (100) crystal plane is observed. With such an arrangement, it is possible to secure homogeneous deposition/dissolution reactions and hence uniform charging/discharging cycles of the battery and suppress the dendrite deposition of lithium metal so as to improve the cycle life of the battery. A substance containing a lithium halide is preferably used for the surface film. According to the Patent Document, the use of a solid solution of at least a lithium halide selected from LiCl, LiBr and LiI and LiF is preferable. More specifically, a negative electrode is prepared for a non-aqueous electrolyte battery by immersing a lithium sheet having preferential orientation of the (100) crystal plane that is prepared by means of a pressure (flat rolling) process in order to produce a solid solution film of at least LiCl, LiBr or LiI and LiF in an electrolyte containing at least either chlorine molecules or chlorine ions, bromine molecules or bromine ions, or iodine molecules or iodine ions and fluorine molecules or fluorine ions. However, with the technique of JP-A-11-288706 that uses a flat rolled lithium metal sheet, the lithium sheet is apt to be exposed to the atmosphere and a film attributable to moisture is likely to be formed on the surface to make active sites exist inhomogeneously so that it is difficult to produce an intended stable surface film and a satisfactory effect of suppressing generation of dendrite cannot be achieved.
Abstracts of the 2000th fall meeting of Electrochemical Society of Japan) 2A24 (2000) and Abstracts of the 41st Battery Symposium in Japan 1E03 (2000) report the effect of a complex of an europium compound or some other lanthanoid-transition metal and imide anions on a lithium metal negative electrode. According to these papers, a surface film of Eu[(C2F5SO2]2]3 complex is formed on the surface of Li metal immersed in an electrolyte prepared by dissolving LiN(C2F5SO2)2 as lithium salt into a mixture solvent of propylene carbonate or ethylene carbonate and 1,2-dimethoxyethane and adding Eu(CF3SO3)3 as additive. However, this method is not satisfactory although it provides an effect of improving the cycle life to a certain extent. Additionally, the use of a lithium imide salt such as LiN(C2F5SO2)2 that is relatively expensive as electrolyte is indispensable and the cycle characteristics are not improved if a complex of some other lithium salt (such as popular LiPF6) transition metal and CF3SO3− ions is added because a complex of transition metal and imide anions is not produced. Furthermore, there arises a problem of high internal resistance of battery because resistance of electrolyte increases if compared with the use of LiPF6 when lithium imide salt is employed as the electrolyte.
Furthermore, a technique of using a carbon material that can occlude and discharge such as graphite or amorphous carbon for a negative electrode to improve the capacity and the charge-discharge efficiency of battery is also reported.
JP-A-5-234583 proposes a negative electrode formed by covering a carbon material with aluminum. With such an arrangement, the reductive decomposition of solvent molecules that are solvated with lithium ions on the carbon surface is suppressed to by turn suppress the degradation of the cycle life of battery. However, the capacity of a battery using such a negative electrode can fall rapidly when the operation cycle is repeated because aluminum reacts with a small quantity of water.
JP-A-5-275077 proposes a negative electrode formed by covering the surface of a carbon material with a thin film of a lithium ion conductive solid electrolyte. With such an arrangement, it is believed that the decomposition of solvent that occurs when a carbon material is employed is suppressed to make it possible to provide a lithium ion secondary battery that can use propylene carbonate. However, cracks that appear in the solid electrolyte due to the change in the stress occurring at the time of inclusion of lithium ions and also at the time of elimination of lithium ions induce degradation of the characteristics. Additionally, reactions do not take place uniformly on the surface of the negative electrode because of the inhomogeneity of the solid electrolyte due to crystal defects and other causes to consequently reduce the cycle life.
JP-A-7-122296 discloses a non-aqueous electrolyte secondary battery having a negative electrode made of a carbon material whose d value at the lattice plane (002) is not more than 3.37 {acute over (Å)} and a non-aqueous electrolyte containing a carbonic acid ester and also a vinylene carbonate derivative. The Patent Document claims that such a non-aqueous secondary battery can suppress decomposition of the non-aqueous electrolyte on the carbon negative electrode and improve the cycle characteristics. However, secondary batteries using vinylene carbonate or a vinylene carbonate derivative still have a room for improvement particularly in terms of high temperature characteristics and cycle characteristics.
JP-A-2000-003274 discloses a secondary battery having a negative electrode made of a material containing graphite and an electrolyte containing a cyclic carbonate and a linear chain carbonate as principal ingredients along with 1,3-propanesultone and/or 1,4-butanesultone by not less than 0.1 wt % and not more than 4 wt %. It is believed that 1,3-propanesultone and/or 1,4-butanesultone contributes to forming of a passive film on the surface of the carbon material so as to coat the carbon material that is turned highly crystalline due to the activity of natural graphite or artificial graphite with the passive film and exerts an effect of suppressing decomposition of the electrolyte without damaging the normal reaction of the battery. However, this technique does not provide a satisfactory film effect and is accompanied by a problem that an electric charge attributable to decomposition of solvent molecules or anions appear as irreversible capacity component to induce a fall of the initial charge-discharge efficiency. Additionally, the resistance of the ingredients of the produced film is high and the resistance rises with time at a high rate particularly at high temperatures. JP-B-5-44946 and U.S. Pat. No. 4,950,768 disclose a method of manufacturing a cyclic sulfonic acid ester having two sulfonyl groups. Compounds having a sulfonyl group are described in JP-A-60-154478 (sulfolane), JP-A-62-100948, JP-A-63-102173, JP-A-11-339850 (1,3-propanesultone and 1,4-butanesultone), JP-A-10-189041, JP-A-2000-235866 (γ-sultone compounds) and JP-A-2000-294278 (sulfolene derivatives). Vinylene carbonate and its derivatives are described in JP-A-4-169075, JP-A-8-45545, JP-A-5-82138, JP-A-5-74486, JP-A-6-52887, JP-A-11-260401, JP-A-2000-208169, JP-A-2001-35530 and JP-A-2000-138071.
However, the prior art does not provide a satisfactory film effect for improving the battery characteristics and still has the problems as listed below.
The surface film formed on the surface of a negative electrode of a battery is closely related to the charge-discharge efficiency, the cycle life and the safety of the battery because of its properties. However, no technique has been known to date for controlling the film for an extended period of time. The effect of suppressing dendrite can be obtained to a certain extent in the initial stages of use when a surface film of lithium halide or a glassy oxide on a layer of lithium or lithium alloy. However, the surface film is degraded and gradually loses its function of protective film as the battery is operated repeatedly. It is believed that this problem arises because the layer of lithium or lithium alloy changes its volume as lithium is occluded and discharged, whereas the film of a lithium halide or the like on the layer hardly changes its volume so that internal stress arises in these layers and the interface thereof. As such internal stress arises, particularly the surface film of a lithium halide is partly damaged to lower the effect of suppressing dendrite.
As for carbon materials such as graphite, the use of such a material does not provide a satisfactory film effect and gives rise to a problem that an electric charge attributable to decomposition of solvent molecules or anions appear as irreversible capacity component to induce a fall of the initial charge-discharge efficiency. Additionally, the composition, the crystalline state and the stability of the film that is produced at this time significantly influence the subsequent efficiency and the cycle life of the battery. Furthermore, the minute amount of moisture existing in the negative electrode of graphite or amorphous carbon accelerates decomposition of the solvent of the electrolyte. Therefore, water molecules need to be eliminated when a negative electrode of graphite or amorphous carbon is employed.
In this way, the film that is formed on the surface of the negative electrode is closely related to the charge-discharge efficiency, the cycle life and the safety of the battery because of its properties. However, no technique has been known to date for controlling the film for an extended period of time. Therefore, there is a demand for newly developed electrolytes capable of forming a film that can realize a stable and satisfactory charge-discharge efficiency at the negative electrode of a battery.